A valve unit 1 is known from US-A-3,810,716. In this publication, a pumping apparatus for a liquid chromatograph is disclosed which comprises a pump head with a bore and a piston for reciprocation within the bore. Check valves are provided at the suction inlet and at the outlet of the pump. The check valves comprise a ball and a corresponding seat and a surrounding housing which confines the movement of the ball. When liquid pushes against the ball from the side opposite the bearing face of the ball, the ball rises from the seat so that the valve is opened. When liquid pushes against the ball from the opposite side, the valve is closed. In the pumping apparatus, the check valves are arranged such they prevent backflow of the sucked in and of the delivered liquid , forcing the liquid being pumped to flow in the desired direction. Thus, during the suction stroke of the pump, the inlet check valve is open and the outlet check valve is closed; during the delivery stroke of the pump it is just the other way round.
The opening and closing behaviour of a check valve, e.g., the distance by which the ball is raised from the seat or the switching time of the valve, depends on the flow rate and pressure of the liquid through the valve, the viscosity of the liquid and other parameters. Thus, the operation of such a valve is not completely reproducible which can result in variations of the flow of the liquid delivered by the pumping apparatus. Such variations are particularly disturbing in high performance liquid chromatography, because they would impair the accuracy of the chromatographic measuring results.
Relative to the above described prior art, it is an object of the invention to provide a valve unit, in particular for a pumping apparatus, which has a higher reproducibility of its opening and closing operation ensuring a more uniform flow of the liquid delivered by the pumping apparatus.